In blind fleas there is often a spine where the eye should be. In one species the spine is rudimentary and there is some black pigment beneath it. It is not impossible that this is the vestige of a once functional eye. In one genus, however, the eye and the spine are both present. Of the fleas belonging to this genus one species is South American and the other European. The latter (_Typhloceras poppei_) is confined to the long-tailed field-mouse. The organs by which fleas keep in touch with the outward world, and with other fleas, are their antennæ. All fleas have antennæ; but unlike those of a moth, a beetle, or a grasshopper, each fits neatly into a groove at the side of the head and can be protruded when desired. This is another adaptation to enable the insect to creep swiftly through a forest of hairs. [Illustration: Fig. 2. Showing a type of (_a_) _genal_ and (_b_) _thoracic_ combs of a flea, on the under part of the head and on the thorax respectively. Analagous combs are found in several other parasitic insects and on the abdominal segments of certain fleas.] The combs which are found on the heads of many fleas are organs of exceptional interest (Fig. 2). They are toothed and horny appendages, which are connected with parasitic habits, for somewhat similar combs are found on several unrelated groups of parasitic insects, as, for instance, on parasitic beetles (_Platypsyllus_) found on the beaver, on insects allied to bed-bugs (_Polyctenes_) found on bats, and on wingless flies (_Nycteribia_) which infest Egyptian and South American bats. The majority of the _Pulicidæ_ have one or more combs with comparatively long teeth. These combs reach their maximum development in the bat-fleas which have no less than eight. Some Australian and South American fleas (_Stephanocircus_) have a helmet-like comb extending all round their heads. These combs are by some supposed to be of service in holding on to the hairs of the host; and, if one may judge from experiments made on live fleas in cotton wool, they are also used in moving forwards through the fur. All the chigoes (_Sarcopsyllidæ_) have a large triangular post-oral process which is more or less curved and probably prevents the flea slipping back as it pushes forward. Bat-fleas (_Ceratopsyllidæ_) have lobes or flaps placed two on each side of the head, which may possibly serve an identical purpose, but whether they do so is not known. These combs may be divided into three groups according to the part of the flea’s body on which they are found. Those found on the head are called genal combs and take the form shown in Fig. 2. There are also combs found on the thorax. The fossil flea (_Palæopsylla klebsi_) described in the previous chapter has one of these thoracic combs. A certain number of fleas also have combs on the abdominal segments.